This invention relates in general to emphasis and de-emphasis circuits and in particular to circuits which reduce noise by altering the spectral content of the signal.
In many adaptive A-D and D-A coding systems the step-size used increases with the level of the input signal. Since quantizing noise increases with the step-size the quantizing noise of such adaptive systems increases with the input signal level, an effect known as noise modulation. The effects of noise modulation is disturbing in many applications, such as in high quality audio.
It is a characteristic of human hearing that spurious spectral information is much less audible if it is close in frequency to the desired audio signal. If the spurious energy lies far from the desired audio signal frequency it is much more likely to be audible. Thus where the noise level is a function of the input signal level it is particularly important to reduce noise whose frequencies are far from the desired audio signal.
Conventional noise reduction systems have been used to apply adaptive emphasis and de-emphasis to reduce audible noise. One conventional system employs fixed emphasis for boosting high frequency signals and complementary de-emphasis for bucking such signals. When such emphasis and de-emphasis are used to reduce audible noise that increases with signal level and when the predominant signals are of high frequencies, low frequency noise will instead be increased. Fixed high frequency emphasis and de-emphasis are therefore unsatisfactory for reducing such noise.
A well known type of circuit, called "sliding band", reduces audible high frequency noise by way of a filter with a variable corner frequency. As the level of high frequency signals increases, the filter corner frequency slides upwardly to narrow the band boosted and cut. Examples of such circuits are to be found in U.S. Pat. Nos. Re. 28,426, 4,072,914 and 3,934,190.
The sliding of the filter corner frequency depends on both the amplitude and frequency of the input signal. If such a "sliding band" type circuit is used to reduce audible noise that is a function of input signal level, low frequency noise may also be increased when the predominant spectral components of the input signal are at very high frequencies. While such a problem is not as serious as in the case of the high frequency fixed emphasis and de-emphasis, the "sliding band" type circuit is not entirely satisfactory for reducing noise which increases with signal level.